We recently found that blood lymphoid cells from polymyositis patients are often slightly toxic for differentiated muscle cultures. This finding suggests that a cell-mediated immune reaction against muscle plays a role in the pathogenesis of polymyositis. We want to pursue this lead in order to determine the importance of cell-mediated immune reactions against muscle in the production of this illness, and to better understand the mechanism of the toxic effect. Differentiated muscle cultures will be challenged in various ways by lymphoid cells and dilute serum from polymyositis patients and controls. Muscle damage will be assessed morphologically and enzymatically. We also want to produce experimental allergic myositis (an animal model of human polymyositis) in rats, in order to study the effect of lymphoid cells from these animals on our muscle cultures. Such studies should promote understanding and development of our tissue-culture system.